1. Technical Field of the Invention
This invention pertains to an improved method of tuning resonant transmission line structures such as those used in high-dielectric-constant, coupled-resonator wave filters.
2. Description of the Prior Art
U.S. Pat. No. 4,431,977, "Ceramic Bandpass Filter," issued Feb. 14, 1984, to Sokola et al., describes ceramic dielectric block filters using resonant transmission line structures to which the tuning method of this invention may be applied. FIG. 1 illustrates a combline filter (100) comprising three transmission line resonators (101, 102, and 103), each having its inner conductor short-circuited to its outer conductor at one end and loaded by a capacitance (e.g. 111, 112 113) at its other end. Electrical signals couple to the filter at an input pad (106) and from the filter at an output pad (107).
FIG. 2a shows an electrical equivalent circuit of the filter of FIG. 1. FIG. 2a represents the equivalent circuit of the combline filter depicted in FIG. 1. Since resonators 101, 102, 103 in FIG. 1 are grounded on identical ends, the equivalent resonators 201, 202, 203 in FIG. 2a are also grounded on the same ends. Also, capacitors 211, 212, 213 in FIG. 2a represent loading capacitors 111, 112, 113 in FIG. 1. Capacitors 241, 242 of FIG. 2a represent capacitances formed by input pad 106 of FIG. 1. Also, capacitors 251, 252 of FIG. 2a represent capacitances formed by output pad 107 of FIG. 1. The input signal at input point (206) couples through a capacitive divider (241-242) between the metallization of pad 106, disc 111, and ground 122. Each resonator (201, 202, and 203) is a transmission line of slightly less than one-quarter wave electrical length; loading capacitors (211, 212, and 213) provide sufficient capacitance to resonate the transmission lines. Output couples from the filter through a similar capacitive divider (251-252) to an output point (207). The response of the filter depends on the electrical parameters of the resonators, on the coupling between resonators, and on the input and output loading. As FIG. 2b illustrates, coupled line filters have also been realized as interdigital structures, in which the relative positions of the short-circuited ends and capacitively-loaded ends of the transmission lines alternate. For example, compare the center resonators (202 and 202') of the two equivalent circuit. Further note resonator 202 of the combline filter in FIG. 2a is grounded on the same side as the other resonators of the same filter, whereas resonator 202' of the interdigital filter in FIG. 2b is grounded on the opposite side as the other resonators of the same filter. Also, compare the input points (206 and 206') of the two equivalent circuits.
FIG. 3 is a cross-sectional detail of a typical prior art dielectric block resonator. The structure may be fabricated from a block that is plated with a conductive coating on its exterior surfaces and on the inner surfaces of the holes. The conductive coating forms both the inner conductor (331) and outer conductor (332) of a transmission line that is short-circuited at one end and capacitively loaded at the other end. A plated region in the form of a conductive disc (311), which is joined to the inner conductor and spaced from the ground plating (322) by a gap (321), provides the capacitive loading. The dimensions of the resonator structure and the dielectric constant of the filler medium (333), which is typically a ceramic such as barium titanate, determine its characteristic impedance and resonant frequency. In FIG. 3, the diameter of the inner conductor of the resonator is designated C, the width of the outer conductor of the resonator is designated D, and the length of the resonator is designated L. The loading capacitance allows the structure to resonate at a frequency slightly below that at which it has one-quarter wave electrical length.
Design of a filter generally requires that each resonator have a specified resonant frequency and coupling to adjacent resonators. Manufacturing tolerances in the dielectric constant and in the physical dimensions require that production filters be tuned after fabrication. Coupling has been adjusted by various prior art methods, including slotting the dielectric between resonators or modifying the conductive plating pattern between the capacitively-loaded, high-impedance ends of the resonators.
Prior art tuning methods have proven inconvenient in production. One method is to provide a grounded tuning screw that protrudes into the dielectric towards the inner conductor in the vicinity of its high-impedance end. Rotation of the screw varies its insertion, which changes the capacitive loading to the resonator. Tuning screws are unsuitable for many practical applications because of their bulk and mechanical instability.
Another method is to abrasively remove conductive material to decrease the loading capacitance and thereby raise the resonant frequency. Because the method cannot lower the frequency, the resonator must be designed so that it will be manufactured below the required value, taking into account production tolerances, and then trimmed higher to the target frequency.
Other methods have addressed bidirectional tuning, that is, the ability to tune either higher and lower in frequency. U.S. Pat. No. 4,157,517, entitled Adjustable Transmission Line Filter and Method of Constructing Same, describes a method of tuning a stripline resonator by removal or addition of ground plane material covering its high-impedance end.
A presently copending U.S. Patent application entitled Adjustable Electronic Filter and Method of Tuning Same Ser. No. 081264, filed July 31, 1987), which is assigned to the assignee of this application, describes another bidirectional tuning method. Removing conductive material from the capacitive loading element tunes a resonator higher in frequency; removing material around part of the circumference of the inner conductor where it joins the outer conductor plating causes the equivalent inductance of the short-circuit to increase, which lowers the resonant frequency. If the resonator has been fabricated with some conductive material omitted, or if some has been removed, conductive material such as metallic paint may be applied to tune in the opposite directions.
These prior art methods suffer drawbacks that make production tuning inefficient: they require that access be available to opposite ends of the resonator according to the direction of tuning, that two steps be carried out (conductor removal and addition), or that all resonators be designed below target frequency. Tuning of interdigital resonators, which have alternating orientations, requires that both ends be available if the same tuning mechanism is to be used. Furthermore, no prior art method has provided effective means to vary tuning sensitivity, that is, the change in frequency for an incremental change in the amount of conductive material.